Austen m



June 1% 11-927.

A. M- CURTIS OSCILLATION GENERATING AND SUPPLY SYSTEM Filed Dec. 8, 1923Patented June 14, 1927.

UNETED STATES innate earner caries.

AUSTEN M. CURTIS, OF EAST ORANGE, NEW JERSEY ASSIGNOR TO WESTERN ELEC-TRIO COMPANY, INCORPORATED, OF NEW YORK, N.

YORK.

Y., A CORPORATION OF NEW OSCILLATION GENERATING AND SUPPLY SYSTEMI.

Application filed December 8, 1923.

This invention relates to oscillation generating and supply systems, andparticularly to apparatus adapted to produce low frequency oscillationsof very pure wave form.

Vacuum tube or other electric discharge oscillators as hithertoconstructed have been unable to produce oscillations within the lowfrequency range extending from zero to a few hundred cycles per secondwithout accompanying harmonics of relatively large magnitude. In certainfields as, for example, in testing telegraph conductors, cables, etc.,it is highly desirable to be able to produce oscillations of the orderof 4 to 60 cycles per second without accompanying harmonics, themagnitude of which is greaterthan a few per cent of that of thefundamental. Filters at such frequencies are impracticable because oftheir excessive size and cost. I

In vacuum tube oscillators of the usual type, there is a feedback of,alternating current from the output to the input circuit of the tube.The oscillations thus supplied to the input circuit operate by virtue ofthe amplifying property of the tube to set up amplified oscillations inthe output circuit thus augmenting the alternating current in the outputcircuit and consequently augmenting also the oscillations fed back tothe input circuit. This process continues until the oscillations buildup to a limiting magnitude determined by the amplification afforded bythe tube and the losses in the system. Beyond this limiting magnitude,an additional increment to the oscillations in the input circuit wouldnot set up in the output circuit an alternating current incrementsuflicicnt'to compensate for the consequent increments in alternatingcurrent energy dissipation occurring in the entire systeml The processjust described involves large all variations in the electromotiveforceimpressed on the grid circuit and large variations in space current inthe plate circuit. As soon as the grid becomes positive with respect tothe cathode, a space current is drawn to it from the cathode thusrendering unsymmetrical the resulting grid potential Wave, since thepositive potential peaks or half cycles are diminished. .Accor'dir flythe non-symmetrical grid poten in site I comprises both save Serial No.679,316.

of the fundamental frequency and a series of waves of even harmonicsthereof, will give rise in the input circuit to an ampli- 'fied wavewhich is by no means sinusoidal but contains the same harmonics as thewave in the input circuit. Obviously, the larger the input current thegreater will be this distorting effect.

The large variations in space current involve operating the vacuum tubefrom a point at or below the lower knee of its wellknown grid potentialplate current characteristic to a point at or above the upper knee ofthis characteristic. It follows that faithful repetition of the inputwave form in the amplified output wave is no longer secured andadditional distortion is produced.

Particularly at low frequencies are the effects of distortion mostdiflicult to avoid. The frequency determining circuit, at suchfrequencies, includes a relatively large inductance. An air coreinductance, of the magnitude required, is not only excessive in size butinvolves distributed capacities which defeat its purpose. Cores ofordinary-magnetic materials have been used and have caused additionaldistortion for the reason that, as previously explained, alternations ofthe oscillator build up to a large limiting magnitude determined by ornegligibly small magnitude. This is accomplished primarily by a specialtransformer coupling the frequency determining circuit and the gridcircuit. This transformer has a magnetic core which at very lowmagnitude of current in the frequency determining circuit experiences alow flux density. The core therefore produces a certain core loss whichmakes its presence felt as an efit'ec 'e resistance in the frequencydete o circuit. With increas ing magni J hf current this density;

this core loss resistance rises much more rapidly than the voltageacross the coil and shortly reaches a point where the loss is sostabilize'at a point determined by the rapid increase in energydissipation in the amplifier itself and in its electrode circuits.

In order to minimize the distortion occasioned by the curved L;Echaracteristic, a-large resistance is connected in series in the spacecurrent circuit and this tends to strai 'hten the characteristic andmake approximately linear that portion of the characteristic to whichthe'operation .of the tube isrestricted.

The supply of very low frequency oscillations from an oscillator to aload circuit involves an additional difficulty. For a transform-er whichwill effectively transfer these very'low frequency oscillations, amagnetic core is highly desirable to enable the necessary inductance tobe obtained Without coils of immoderate size. This transformer cannotbedirectly connected in the space currcnt'circuit since thatwould involvepolarization of the magnetic core which would consequently entaildistortion and production otharmonics. It might be supposed that theload connection could bemade to the oscillator through a series blockingcondenserbut this would involve an inordinate ly large capacity and theinherent tendency. of the capacity element to discriminate against thelower frequencies rather than. against the higher harmonic frequencies.A

feature of this invention consists in neutralizing the polarizing effectof space current characteristic. of the inventionlare pointedout withparticularity in the appended claims. The invention itself, however,.both

as tothedetails of its organization and its.

mode of operation will be best understood by reference to the followingdescription taken in connection with the accompanying drawingrin whichFig. lillustrates schematically an oscillator supply circuit embodyingthe invention, and Fig. 2 shows the transformer which couples thefrequency determining circuit to the grid circuit.

lfleferring to Fig. l, a vacuum tube oscillator comprising, one or morevacuum tubes l'of'well-known type is shown with a space urrentsoiu'ce l3'ro'nnected' th rougha series resistance Connected across the-plate andcathode is an alternating current path includinga capacity element 4 anda high resistance 5"from which is tapped a feedback conductor 9. Avariable capacity 6' and a primary winding of a transformer 7 constitutea frequency determining circuit upon which oscillations are impressedthrough the common ground conductor 8 and the feedback path 19 whichincludes high resistance l0.

'lrausformer 7 is specially desi ned to transmit low frequency currentse ectively and to be very efficient when operated at low core fluxdensity. For this purpose a permalloy core is especially suitable with amagnetic circuit closed except for a small stabilizing air gap. For adescription of the characteristics of permalloy, reference is made to anarticle by Arnold and Elmen. published in the Journal of the FranklinInstitute, May, 1923. that thecore may comprise an iron-nickel alloy asdescribed in the Arnold-Elmen article having at least nickel and thebalance iron, or in whichthe nickel is within a few per cent of 78.5% ofthe nickel-iron content, which is heat treated to develop It issufficient to state ion therein a permeability greaterthan iron atmagnetizing forees'in the entire range below 2/ 10 gauss. While thefluxdensity is below the magnitude where the core losses are not' toogreat, a potential variation in the frefrequency determining circuitwill maintain a the potential variations at their original value. lVithhigher flux, densities occasioned by larger eurrents, the core lossessoon rise to a point beyondwhieh" theyrcan" no longer bemade up by'theamplifier. Thiis extrao dinarily rapid increase in core loss withincreasing flux density is peculiarly.

characteristic of certain magnetie materials suchas permalloy, It,moreover. occurs'at a flux densitywhich is comparativelyulow thusenabling the oscillator to stabilize ata correspondingly low currcnhamlitude.

Resistance 10 of the feedbac path 'restricts oscillations fed backtosm'alhmaginitudes. Its variable connection to resistanice 5 permitstheelectromotive force has pressed on the gridcircuitto beregnlated tubeitself. The alternating current paths through resistances and 10 are ofseveral times the tube internal plate circuit impedance. It follows thatthe ((JD'llJiilOCl tube and extcmalcircuit characteristic is fairlvlinear over a considerable range of grid potential variations. Theeffect of this is to permit the amplifier to amplify impressed.alternating input waves without substantial distortion.

The oscillations in resistance 5 set up a potential difference in theinput circuit of an'amplifying system comprising one or more amplifiers12 permanently connected in circuit and one or more additionalamplifiers 13 adapted to be connected in parallel as circumstancesdictate. The output circuit of these amplifiers is associated with aload circuit 14 by means of a magnetic core transformer 15 designed toelliciently transmit alternating current at low frequencies. As has beenpreviously pointed out, it is desirable to prevent direct cur rentpolarization of the core by the priniary winding and to do this withoutinterposition of a blocking capacity. For this purpose, space current ofthe amplifier 12 is caused to traverse a resistance 16. The primarywinding of transformer 15 is in a shunt path to resistance 16, the pathalso including a neutralizing source of voltage 17. Any tendency forspace current to flow through the primary winding is overcome byoppositely-poled voltage source 17, variable resistance 18 enabling anexact balance to be had. This circuit permits low frequency currents tobe transmitted very effectively and by reason of leakage inductance ofthe transformer, causes some discrimination in favor of low frequencycurrents as against those of higher frequencie.

A double-pole switch 19 enables the additional tubes 13 to be cut in asdesired, its left-hand contact closing their cathode-heating circuit. Atthe same time, the righthand contact of the switch short-circuits aportion of resistance 16 to permit the neutralizing balance to bemaintained with the increased space current.

Fig. 2 illustrates diagrammatically cer tain details of one form whichtransformer 7 may take. Flat side strips 20 of permalloy are stacked upalternately with end strips 21 of similar material. This rectangularframe core may be held in assembled position by bol ts 22,. orpreferably clamps. Strips 23 of the same material cX- tend down betweenthe lower end strips 21. to the margin 27, as indicated in dotted lines.is so far described, there is a space factor of in the core structureexcept for the overlapping corner portions which have substantially nounoccupied space. However. the central leg! of the core is filled byinserting between the alternate long; strips, shorter strips 23 whichextend only to the margin 26, there abutting corresponding strips 2!. 0nthe central leg of the core are wound primary winding; coils 24: andsecondary coils 25.

In an actual oscillator designed for a working range of approximately 4to 600 cycles, the following circuit constants and characteristics wereinvolved: Amplifier 1., two d estern Electric V type tubes in par allel.resistance 10, 700,000 ohms; total resistance elen'ient 360,000 ohms;resistance 2, 24,000 ohms; capacity 4. 8 hi. F.; capacity 6, variable upto 2 M. E; voltage of source B, 250 volts.

The details of transformer 7 are approximately as follows: width ofstrips 20, 21 and 23, thickness of these strips 6 1?!5; overall lengthof strip 21, 4%; of

str i 20, 6; air gap 28 at end of central core leg approximately 10mils. The core structure was piled up until itattained a thickness ordepth perpendicular to the plane of the paper of about 2". Under thesecircumstances, the flux density of the pennalloy core ranges from about200 at the loweset frequency to about 4: gauss at 500 cycles. lVith sucha circuit, the magnitude of harmonics at no time exceeded 2%% to 3% ofthe magnitude of the fundamental, whereas with the best previouslydesigned oscillators working" in this frequency range, the harmonicsrose to about 80% of the fundamental at the lower fre-- quency setting.

Frequency variation of the circuit illustrated is obtained by varyingcondenser 6. This does not, however, give a very large frequency range.In order to increase the range resort may be had to the expedient ofdividing the windings of transformer 7 into a plurality of primarysections 2 1- and secondary sections 25, as illustrated in Fig. 2. Thesections of each winding may be connected in parallel or in parallelseries combinations all. in well-known manner to vary the effectiveinductance accordingly. Illustration of this well-known feature isomitted in the interest of simplicity of disclosure.

In the transformer, certain constants of which have previously beengiven, the nine primary winding sections each comprise 1,800 turns ofNo.3'l3single silk covered copper wire, the two secondary sections areeach wound with 3,300 turns of No. 39 double silk covered copper wire.

'lhroughout this disclosure, the term vacuum tube has been used in ageneric sense to denote electron relays or electric discharge devices ofany character whatever, regardless of their evacuation or gaseouspressure or of the manner of energizing their cathodes or controllingtheir respective impedances. It may be noted that the amplifiers l2 and13 actually used with the circuit described were Western Electric 0 typetubes.

It will, of coiu'se, be appreciated that the magnitudes given are merelyillustrative of one very useful circuit, and that not only thesemagnitudes, but the circuit structure itself and the apparatus, may bewidely varied Without departing from the spirit of the invention asdefined in the appended claims.

What is claimed is:

1. An oscillator for producing frequencies of the order encountered inMorse telegraph practice comprising an electron relay having input andoutput circuits coupled by a transformer, said transformer having a corecomprising a magnetic material having permeability higher than that ofiron at magnetizing'forces of 2/10 gauss or less and comprising at least30% nickel and the balance iron, said core operating to limit theamplitude of the energy supplied by said output'circuit to said inputcircuit.

2. An oscillator comprising an amplifier having output andinput'circuits coupled in feedback relation by a transformer, saidtransformer having a core of magnetic material comprising nickel andiron having developed therein by heat treatment permeability greaterthan iron at magnetizing forces in the entire range below 2/10 gausspling said circuits comprising a transformer having a core of magneticmaterial having higher permeability than that of iron at magnetic forcesof 2 /10 of a gauss or less and including nickel and iron, the nickelbeing within a few per cent of 7 8.5% of the nickel-iron content, andaresistance limitin the current supplfed to said primary win .ing to avalue corresponding to a. COI'BfilIX density not exceeding 200 gauss.

4. Means for producing low frequency oscillations of high urity of waveform, a load circuit, means or augmenting said oscillations andtransferring themto said load' circuit without introducingdistortion,comprising a vacuum tube amplifier directly connected to saidoscillation producing means, a magnetic core transformer having aprimary winding'directly connected to the output circuitof saidamplifier whereby voltages of all frequencies are impressed on saidwinding without substantial discrimination, and means to neutralize thetendnc of the vacuum tube direct current to pc arize said core, saidtransformer having a secondary winding connected to said load circuit.

In witness whereof, I hereunto subscribe" my name this 1st day ofDecember, A. D. 1923.

AUSTEN M. CURTIS.

